1. Field of the Invention
The present invention relates to a coating method, and in particular to a suitable coating method using an extrusion type coating device (extrusion dies, which will be hereinbelow denoted simply as "dies") suitable for forming a multi-layer coating film on a base material.
2. Discussion of the Background
Conventionally, it is known to extrude multiple coating liquids onto a base material to form a multi-layer coating film on the base material using an extrusion-type coating device (such inventions as disclosed in Japanese Laid-Open Patent (JP-A) No. 63-88080 and No. 2-251265).
According to this conventional technology, one upstream lip and two or more downstream lips are incorporated in a coating apparatus that has a plurality of slots formed between these lips; coating liquid can be extruded through the slots. The coating liquid may be of the same or different in composition and is extruded, through respective slot parts, onto a base material fed in a first direction from the upstream lip toward the downstream lips. Thereby, the liquid coated on the base material at the upstream slots is overcoated by the other liquid at the downstream slots. In this way, a coating film of multi-layer structure is formed on the base material.
FIG. 5 is a sectional view showing a condition in which a two-layer coating film is formed by a coating device 1 comprising an upstream lip 2, a first downstream lip 3 and a second downstream lip 4, and a first slot part 5 defined between the upstream lip 2 and the first downstream lip 3, and a second slot part 6 defined between the first downstream lip 3 and the second downstream lip 4. Coating liquid 7 for the lower layer extruded from the first slot part 5 and coating liquid 8 for the upper layer extruded from the second slot part 6 form a coating film 10 having a two-layer structure composed of a lower coating film 10A and an upper coating film 10B on a web 9 as a base material.
However, even though such a conventional coating device as mentioned above is suitably used, a satisfactory coating film cannot be formed. In certain cases, streak-like unevenness is developed at equal pitches along the coating direction, and in other cases, relatively large width streak-like bright-and-dark unevenness caused by there even thickness is developed also along the coating direction.
Most of the prior art mentioned above has been concerned with the shape of the tip end of a lip (coating work surface), and in particular the curvature of a lip curved convexly toward the web. Japanese Laid-Open Patent (JP-A) No. 63-88080 discloses a process in which differences in the radii of curvature of the downstream lips is set in a predetermined range in order to obtain satisfactory coating, and specifically, a height difference between the lips is set in a range of -300 to +300 .mu.m. However, in the coating conditions (such as a thickness of a coating film) stated in the embodiments disclosed in the Japanese Laid-Open Patent (JP-A) No. 63-88080, the height difference between the lips should actually be set in a more limited range.
In more detail, if the height difference between the downstream lips in the two-layer coating die is adjusted so that the ratio between the film thickness h of the lower coating film 10A which is extruded from the first slot part 5 and which is not yet dried, and a gap G defined between the downstream end of the coating work surface (lip surface) 11 of the first downstream lip 3 and the web 9, at the position a, is set as follows: EQU h/G&gt;0.5, h/G&lt;0.4
the coating film 10 becomes uneven.
That is, in the case of h/G&gt;0.5, at least a part of an interface 12 at the boundary between the lower layer of coating liquid 7 and the upper layer of coating liquid 8, in the widthwise direction of the dies separates from the above-mentioned downstream end position a of the first downstream lip 3 in the direction A, as shown in FIG. 6A, and accordingly, meniscus unevenness is developed in the coating film 10 in the coating widthwise direction.
Further, in the case of h/G&lt;0.4, an interface 12 between the lower layer of coating liquid 7 and the upper layer of coating liquid 8 separates from the downstream end position a of the first downstream 3 in the direction B as shown in FIG. 6B, and accordingly, relatively large width streak-like bright-and-dark unevenness caused by there even thickness or mottled unevenness is developed in the coating direction. It is noted that reference numeral f of FIG. 5 denotes the film thickness of the upper layer of coating film 10B which has not yet dried.